In a small bowl, combine the cornstarch, 1/2 cup of the milk, and 1/4 cup of the brown sugar. In a small bowl, lightly beat the egg.

In a medium saucepan, combine the remaining 1 cup milk and 1/4 cup brown sugar, the peanut butter, and the salt. Bring to a boil over medium heat. Whisk the cornstarch mixture into the hot milk mixture and cook, whisking, until thickened, about 5 minutes.

Gradually whisk some of the hot milk mixture into the beaten egg, then whisk the warmed egg mixture back into the hot milk mixture and bring to a boil over medium heat. Reduce the heat to low and cook, whisking constantly, until the mixture is thickened, about 2 minutes. Remove from the heat and stir in the chocolate liqueur. Let cool to room temperature.

Meanwhile, place the water in a small heatproof bowl, sprinkle the gelatin over, and let stand until softened, about 4 minutes. Set the bowl over a small saucepan of simmering water and stir until the gelatin dissolves, about 3 minutes. Set aside to cool to room temperature.

With an electric mixer, beat the half-and-half and yogurt until foamy. Gradually add the honey and beat until thickened. Beat in the dissolved gelatin until well combined.

Fold the half-and-half mixture into the peanut butter mixture. Spoon the mousse into 4 serving bowls and chill until set, about 2 hours.

To serve, drizzle each serving with the melted chocolate and sprinkle with the peanuts.

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